How do I make it where it place the item on the grill and cooks it?

ExampleGillcook

They click the grill then .it cooks it

There is most likely a click detector, that when pressed with make the food visible on that grill.

Workspace.Grill.clickdetector.MouseClick:Connect(Function()
Pan.Transparency = 0
-- or they might of cloned it.
Food:Clone()
-- Change location/parent

End)

Then finally they added some particle effects for the added touch.

Then again they could of done it some other way but this is how I would do it.
Clearly my eyes need to be checked I didn’t notice it was a decal

But its mostly the same use a loop to change the decals Transparency or the color of it all together.

Have multiple decals in place, or different roblox asset ids to use. As time goes on (you can just use the wait system), replace the decal ID of the food.

Following on with what kyle had said, use the MouseClick event. However, for the sake of convenience and no need to name groups/parts differently, you can place the script in the part/group and use script.Parent to find the grill.